1. Field of the Invention
The present invention generally relates to an MR (magnetic resonance) imaging apparatus for acquiring an MR image of a biological body under medical examination by employing a plurality of surface coils functioning as NMR (nuclear magnetic resonance) signal-receiving coils. More specifically, the present invention is directed to an MR imaging apparatus for selectively energizing these surface coils with respect to a portion of the biological body to be imaged.
2. Description of the Related Art
Very recently, so-called "multiple surface coil type MR imaging apparatuses" have been developed in which a plurality of surface coils functioning as NMR-signal receiving coils are arranged around a biological body under medical examination and only surface coils required for the MR imaging are energized or brought into active conditions so as to improve the S/N ratio. These multiple surface coil type MR imaging apparatuses are known from, for instance, U.S. Pat. Nos. 4,825,162 to Roemer; 4,924,868 to Krause; 4,859,947 to Boskamp.
In such a multiple surface coil type MR imaging apparatus, only one receiver coil positioned at an imaging portion of a biological body is manually selected for energization prior to an MR imaging operation.
However, since the setting position of the surface coil to be energized is previously determined with respect to the biological body, the imaged portion of the biological body is accordingly restricted. As a result, no multi-slice MR imaging operation is carried out in which the NMR signals derived from a large number of slice portions should be acquired.
On the other hand, if a plurality of surface coils having, in general, high sensitivities are simultaneously energized, such high-sensitive surface coils which do not contribute the MR imaging operation are brought into active conditions. As a consequence, the noises generated from the biological body functioning as a conductor are induced in these surface coils, so that the S/N ratio of this case is considerably lowered, as compared with the S/N ratio under such a condition that either a single surface coil, or only limited number of surface coils are energized for MR imaging purpose.